You are assuming the price of Bitcoin is going to stay forever at today's price?
It seems less about the price and more about the difficulty. If you click on the chart and change it to "coins generated" and look at it over 24 months, it basically shows that you will be generating next to nothing by month 12.
you are 100% in asking the question and difficulty change play a big part in this. The thing I am taking into consideration - for the difficulty to increase by 10% or 20% per month the network has to add that % hashing power. Now with an estimated 16-17Eh in the network... calculate the number of S9s needed to do that. It will plato imo but continue going up. How much ? Who knows.
edit: also keep in mind that other SHA-256 coins take some of the hashing produced.